nerdkits digital calipers dro we recently got a smithy 1220xl lathe mill bo machine but we quickly found that the 0 042" resolution ticks on the milling head s z axis were a big limitation raspberry pi – driving a relay using gpio – susanet great post knocked one of these up last night – works a treat i was impatient and only had maplin to hand so i used n18aw relay a cheap diode a cheap 1 2kohm resistor and a 2n3904 transistor the best 4 transistors to keep in your parts kit bald w hen your project needs a transistor there are tons of choices which makes answering the question “which transistor should i use or ” a daunting task